This invention relates to an optical disk recording thereon compressed data of an image, and to an optical disk reproduction apparatus for reproducing the image data from the optical disk.
A so-called "CD-ROM" is a typical example of those systems which reproduce digital data by using an optical disk. The CD-ROM records data for a computer on an optical disk having the same physical format as CDs for audio use, and has the data format to be next described. A data string recorded on the optical disk comprises the smallest unit referred to as a "frame", and each frame contains digital data such as sync data, subcode, main information and an error correction code.
Further, the CD-ROM employs the sector structure in which 98 frames (2,352 bytes) are gathered into 1 sector, and each sector comprises a 12-byte sync data, a 4-byte header data representing an address and a mode, a 2,048-byte digital data and a 288-byte error detection/correction code.
On the other hand, a system comprising the combination of an inter-frame prediction with orthogonal transform, quantitization and variable-length encoding is well known as an encoding system of moving picture, and an MPEG system of the ISO (International Organization of Standardization) is also based on this system. In the case of MPEG2, for example, the bit stream of the encoded image data is divided into six hierarchical layers, i.e. a sequence layer, a GOP (Group of Pictures) layer, a picture layer, a slice layer, a macro-block layer and a block layer. Among them, the GOP layer contains three kinds of data, that is, an I picture encoded from the information alone without using inter-frame prediction, a P picture generated by executing prediction from the I picture or P picture and a B picture generated by bidirectional prediction. The sequence layer comprises a GOP containing image data starting from the I picture and obtained by gathering the I picture, the P picture and B picture into one group, and an SH (Sequence Header) added to the leading part of the GOP.
When moving pictures are converted to compressed image data by high efficiency encoding, there is known a system which encodes the signal by reducing a compression ratio for scenes having vigorous motion, or in other words, at a high transfer rate, and by increasing the compression ratio for scenes having small motion, or in other words, at a low transfer rate. The variable transfer rate compressed image data so encoded can reduce image deterioration due to compression in comparison with compressed image data of a fixed transfer rate obtained by fixing the compression ratio at a mean value.
An apparatus for recording the compressed image data having the variable transfer rate or the fixed transfer rate into the optical disk such as the CD-ROM and reproducing the data has been announced already.